In magnetic recording media technology, higher density recording capability has been demanded. As one of the means meeting this demand, it is known to smoothen the surface of the magnetic layer, for example, to smoothen the surface by further fining the particle sizes of the ferromagnetic powder which is used in the magnetic layer.
However, when the surface associated with a magnetic layer is smoothened, the friction coefficient due to the contact between the magnetic layer and an apparatus during running of the magnetic recording medium increases. As a result, the magnetic layer of the magnetic recording medium tends to be injured in a short period of time of running or the magnetic layer tends to be peeled off.
In an effect to solve this problem, lubricants such as fatty acids, fatty acid esters, hydrocarbons, silicone compounds, and the like, have been used.
Video tape recorders (VTR) and flexible disk drive apparatus, such as personal computers and word processors have been popularized and generally used. Also, magnetic recording media have been used in a wide range of environmental conditions such as under a low temperature or under a high-temperature high-humidity condition. Accordingly, a magnetic recording medium must be sufficiently stable such that the running durability thereof is not changed under various conditions that can be expected. However, the stability of the magnetic recording media using conventionally known lubricants still remains insufficient for fully attaining this object.
For instance, silicone compounds have been used in effects to solve the aforesaid running durability problem owing to the specific surface improving faculty and the excellent fluidity imparted by silicone compounds. For example, various fatty acid-modified silicone compounds were investigated for improving the retention of the lubricants on the magnetic layer of a magnetic recording medium as shown in JP-B-63-29333 (the term "JP-B" as used herein means an "examined Japanese patent publication), JP-A-60-1623, JP-A-56-169223, and JP-A-57-37735 (the term "JP-A" as used herein means an "unexamined published Japanese patent application."
However, when the fatty acid-modified silicone compound is a saturated fatty acid-modified silicone, the melting point (softening point) thereof becomes relatively high and the lubricating effect at lower temperatures is reduced. In comparison, when the silicone compound is an unsaturated fatty acid-modified silicone, the fluidity thereof is excellent, but when this type of silicone compound is applied to a very smooth magnetic recording medium which is excellent in high-density recording aptitude, the coefficient of static friction is increased, which is undesirable for the magnetic recording medium.
For example, JP-B-63-29333 discloses organic silicone compounds shown by following formula; ##STR2## wherein R' represents a saturated or unsaturated hydrocarbon group having from 7 to 21 carbon atoms, m an n represent such integers as 1.ltoreq.m&lt;150, 0.ltoreq.n, m +n.ltoreq.1000, and m/n=1/1000 to 1/1,
When R.sub.1 in the aforesaid silicone compound is selected to be a saturated or unsaturated straight chain hydrocarbon group having from 7 to 21 carbon atoms and the compound used as a lubricant, the lubricating property and the abrasion resistance of the magnetic layer are improved and also the running stability and the durability of the magnetic layer are improved. However, in the case of the saturated fatty acid-modified compound, the melting point (softening point) of the lubricant is relatively high. As a consequence, the lubricating effect at lower temperatures is reduced. Also, in the case of the unsaturated fatty acid-modified compound, the fluidity is excellent but the coefficient of static friction is increased.
Thus, other organic silicone compounds also have been proposed. Namely, the compounds represented by following formula (II) as described in JP-B-49-14249 and the compounds represented by following formula (III) described in JP-A-50-32904 have been proposed. ##STR3## wherein R and R' each represents a hydrocarbon group having from 7 to 26 carbon atoms and n represents an integer of from 1 to 20. ##STR4## wherein R and R' each represents a hydrocarbon group having from 8 to 18 carbon atoms; each R" represents a hydrogen atom or a methyl group; 15.gtoreq.m.gtoreq.1, and 16.gtoreq.(n.sub.1 +n.sub.2).gtoreq.2.
However, the use of the compound shown by the above-described formula (II) is undesirable since not only because the durability of the magnetic recording medium is not improved but also because the compound has a low compatibility with a binder which causes an excessive uncontrolled blooming phenomenon. Although not completely understood, this phenomenon can be attributed to the fact that the bond between the hydrocarbon bond and silicon is an ether bond which has poor polarity.
Also, it has been found that the use of the above-described compound represented by formula (III) is undesirable since the coefficient of kinematic friction of the magnetic recording tape with the head drum of a video tape recorder is increased at a heavy load to sometimes cause creak. Also, the durability of the magnetic recording medium is not significantly improved by the aforesaid compound. Although not completely understood, this result considered to be caused by the presence of the hydrophilic alkylene oxide chain in this silicone molecule.
Thus, as the result of investigations for solving the aforesaid problems, another proposal that has been offered involves a magnetic recording medium comprising a non-magnetic support having thereon a magnetic layer containing the organic silicone compound represented by following formula (IV) together with a ferromagnetic powder and a binder as described in JP-B-56-26890. ##STR5## wherein R, R', and R" each represents a saturated or unsaturated hydrocarbon group having from 7 to 21 carbon atoms, m represents an integer of from 1 to 100; n represents an integer of from 0 to 250, m+n.gtoreq.300, and m.gtoreq.n/5.
It has been found that by the aforesaid technique, a magnetic recording medium is obtained having a stable running property, an excellent abrasion resistance, a high durability and experiencing considerably less blooming phenomenon.
Recently, with the development of magnetic recording, recording of high image quality and high tone quality has been highly demanded. For meeting these demanded requirements, it has hitherto been practiced to fine the particle sizes and packing density of the ferromagnetic powder used in the magnetic recording medium.
Also, with the recent great increase in the use of magnetic recording media, techniques are needed which enable magnetic recording media to be produced at a lower cost. One example of such a technique involves the employment of a plural magnetic layers. In the plural magnetic layers, since the upper layer is imparted high image quality characteristics and the lower layer is imparted high tone quality characteristics, adequate amounts and types ferromagnetic powders can be independently selected for each of these layers, which facilitates achieving the goal of increasing density. Also, in the plural magnetic layers, other proper additives can be independently selected for in each layer so that only the particular properties desired in each layer need be provided, whereby the magnetic recording medium can be produced at a lower cost.
At present, conventional thinking considered that a method which improves the surface property of the surface of a magnetic layer is consequently effective for improving the electromagnetic characteristics of the magnetic recording medium. In the case of magnetic recording tape, for realizing the purpose of improving the surface property of the upper layer, the surface property of the lower layer also must be improved. Towards this end, if a binder having a low molecular weight is used in the lower layer to soften the lower layer, the surface property of the lower layer can be improved, whereby the magnetic recording tape having plural magnetic layers has excellent electromagnetic characteristics.
On the other hand, the electromagnetic characteristics of such a magnetic recording medium is a property which has been being evaluated on the presumption that the magnetic recording medium still will have a good running property maintained at an adequate level. Hence for ensuring the maintenance of good running property in the magnetic recording medium, it is necessary that the friction coefficient of the surface of the magnetic layer also be reduced as much as possible.
Accordingly, the good running property is preserved along with the obtained excellent electromagnetic characteristics of the magnetic recording medium by adding an effective amount of a lubricant to the upper layer and also the lower formed on a non-magnetic support.
However, when the aforesaid organic silicone compound represented by the above-described formula (IV) is used as a lubricant for the plural magnetic layers, there are problems are encountered in that a sufficient video output is not obtained, and in low-speed running and high-speed running, the coefficient of friction (.mu. value) observed after 100 running times is increased.